1. Field of the Invention
The present invention relates to the field of current mirrors, particularly as used in integrated circuits.
2. Prior Art
Current mirrors are very frequently used in integrated circuits to set bias currents for various parts of the circuit. Typically the currents of one or more current sources, such as a current source that is independent of temperature or proportional to absolute temperature, is mirrored to various parts of a circuit so that one (or a very few) current sources may be mirrored to numerous sub-circuits for biasing purposes. In other cases, current mirrors may be used in the signal path itself, mirroring a signal current of one sub-circuit to one or more other sub-circuits. Whatever the application of the current mirror, the accuracy and/or sensitivity of the current mirror to such parameters as power supply noise and xcex2 (beta) variation of the transistors used (junction transistors in this example) with process variations and collector current frequently has a very substantial effect on the performance of the circuit. Reduction in such sensitivities can substantially improve circuit performance, or reduce power supply filtering requirements, or both.
By way of example, the well-known PNP current mirror circuit is shown in FIG. 1. The output current IO is:
IO=IIN/(1+(p+1)/xcex2PNP)
Where:
IIN=the input current to the current mirror
p=the area ratio of transistor Q2 to transistor Q1
xcex2PNP=the ratio of collector current to base current for the PNP transistors Q1 and Q2
The current multiplication error is set by the xcex2PNP parameter value. For most cases this parameter has a low value (10 to 50) and is rapidly falling at high collector currents. The output current sensitivity to xcex2PNP variation is:
(xcex94IO/IO)/(xcex94xcex2PNP/xcex2PNP)≅(1+p)/xcex2PNP
The output current sensitivity to power supply voltage variation is:
(xcex94IO/xcex94V+)/IO≅1/VAP
Low voltage, fast settling precision current mirrors and methods. The precision current mirror have first and second current mirrors, each having an input to be mirrored and a mirror output, the current mirrors being coupled so that the mirror output of each current mirror receives part of the input to be mirrored by the other current mirror, the first current mirror also mirroring current for re-mirroring to the input of the second current mirror, and to a precision current mirror output in proportion to the current provided to the input of the second current mirror. Various embodiments are disclosed, including MOS and junction transistor embodiments, and embodiments having increased output impedance.